This invention belongs to the field of ultra-sensitive analysis of gaseous species or more specifically, to spectroscopic methods of analysis such as, for example, Photo Acoustic Spectroscopy (PAS) that benefit from a high intensity light source. PAS has been among the methods of ultra-sensitive spectroscopy popular in scientific research, but to date it has had a very limited impact on the gas sensing industry. Many years of PAS research as an analytical method has resulted in a general understanding of the nature of the photo acoustic effect and of a suitable configuration for photo acoustic cells. Classical PAS (a microphone and a resonant acoustic cell) have reached a high level of operating performance. As evidence of such is the fact that the best results demonstrated in recent years by different groups e.g., [A. Miklos, et. al., Rev. Sci. Instr., 72(4), 1937-1955 (2001); M. Webber, et. al., Appl. Opt., 42(12), 2119-2126 (2003) and V. Kapitanov, et. al., Appl. Phys. B, 90, 235-241 (2008)] are very close and report values for normalized noise equivalent absorption (NNEA) of from 1.5 to 2.5×10−9 cm−1 W/Hz1/2. This means that with such a photo-acoustic cell an absorption coefficient k between 1.5×10−9 cm−1 and 2.5×10−9 cm−1 can be detected with a signal to noise ratio of one using a laser source power of 1 W, provided that the equivalent noise bandwidth of the detection electronics is equal to 1 Hz. However, the sensitive general purpose microphones used in PAS required elaborate acoustic isolation of the sample cells, and low immunity to ambient noise may be a reason impeding its use in noisy industrial environments. Less than a decade ago a quartz tuning fork (QTF) was introduced as a novel photo acoustic sensor, see [U.S. Pat. No. 7,245,380 (2002) and A. Kosterev, et. al., Optics Letters, 27(21), 1902-1904 (2002)]. The QTF is sensitive to the local pressure variation due to the optical absorption between the fork tines, but it has a high immunity to ambient acoustic perturbations coming in the form of plane waves. The method has been named QEPAS—Quartz Enhanced Photo Acoustic Spectroscopy. QEPAS has reached the same performance level in sensitivity as conventional PAS, [see A. Kosterev, et. al., Optics Letters, 27(21), 1902-1904 (2002)], but with the important advantage of a very small sensor size. Surprisingly however, even in 2009, seven years after the invention of QEPAS, one cannot find a single commercial gas detection product based upon QEPAS. Yet even more surprising, there are only a few examples of commercial gas sensors based upon “classical” PAS cells despite the fact that PAS using lasers was introduced as early as in 1968 by Kerr and Atwood, see Applied Optics, 7(5), 915-922 (1968). The explanation is simple—the power of the commercially available laser sources is too low, and therefore the limits of detection on concentration are non-competitive with other methods. Only distributed feedback (DFB) lasers intended for use in the telecommunication industry, which operate in the spectral range from 1260 to 1675 nm (O to U-band), can currently meet the requirements for an industrial gas sensor, i.e., robustness, ease of use, reliability and an affordable price. The wavelength ranges for Telecommunications Optical Bands are as follows:
Wave length in Band NameNanometers (nm)O-band Original1260-1360E-band Extended1360-1460S-band Short1460-1530C-band Conventional 1530-1565L-band Long1565-1625U-band Ultra-long1625-1675
Beyond this range, extended versions of such lasers are available from a limited number of vendors for wavelengths of up to 2350 nm, but at a significantly higher price. The output power of all such lasers is in the range of 10 to 100 mW which results in a noise equivalent absorption (NEA) of 2×10−8 to 2×10−7 cm−1/Hz1/2, both for conventional PAS and also for QEPAS. This corresponds to a noise equivalent concentration (NEC) for many important species of not higher than 0.1 ppmv/Hz1/2, which leaves PAS using telecom DFB lasers entirely out of competition in performance with other ultra-sensitive spectroscopic techniques such as Cavity-Ring-Down Spectroscopy (CRDS) that have a NEA of about 3×10−11 cm−1/Hz1/2. PAS has several important advantages compared to other gas detection methods, which would make it a method of choice provided that the penalty in sensitivity could be overcome. Some of these advantages have become especially attractive with the advent of Quantum Cascade Lasers (QCL) operating in the mid-IR. These advantages include:                PAS is an intrinsically zero baseline technique, no absorption—no signal        High immunity to interference fringes which should permit long time averaging—a big advantage relative to CRDS and ICOS        Inexpensive microphones as detectors, as opposed to cryogenically cooled MCT photo-diodes. No need for low-noise high sensitivity, high linearity and large bandwidth detectors. All intensity monitoring can be done with low-cost detectors, a big advantage in the mid-infrared spectral range.        The smallest detection volume with QEPAS (˜1 cm3 or less) permitting high-speed gas monitoring. For comparison, in CRDS the volume can be ˜20 cm3 and in ICOS it is about 1 liter.        Absence of critical (and costly) optical components in contrast to CRDS and ICOS which require ultra-high reflectivity mirrors.        
It is an object of the current invention to increase the sensitivity of PAS in general, and QEPAS in particular, several hundred times, and thus to bring the PAS performance (NEA) up to the level of other ultrasensitive optical sensing techniques. There are three contributing parts to the NEA of a photo-acoustic gas detector—sensor responsivity R, sensor noise N, and optical excitation power P:
                              N          ⁢                                          ⁢          E          ⁢                                          ⁢          A                =                              N            R                    ⁢                      P            .                                              (        1        )            
The sensor responsivity R has units of V/cm−·W or A/cm−1·W and designates the electrical signal of the acoustic signal transducer (microphone or tuning fork) per unit of optical absorption coefficient and per unit optical power. The responsivity can be increased 10 to 50 times by arranging an acoustic resonator around the acoustic signal transducer, this has been done both in PAS, [see A. Miklos, et. al., Rev. Sci. Instr., 72(4), 1937-1955 (2001); M. Webber, et. al., Appl. Opt., 42(12), 2119-2126 (2003) and V. Kapitanov, et. al., Appl. Phys. B, 90, 235-241 (2008) and in A. Kosterev, et. al., Optics Letters, 27(21), 1902-1904 (2002) and A. Kosterev et. al, LACSEA 2006, Incline Village, Nev., Feb. 5-9 (2006)]. One can say that these acoustic resonance enhancement techniques have been generally optimized and that one cannot reasonably expect a further responsivity increase of no greater than several tens of percents. Equation (1) shows that it is not the responsivity R alone, but its ratio to the r.m.s. noise in the unit frequency bandwidth N that determines the NEA. The fundamental reason for the noise of an ideal acoustic transducer in a quiet environment should be random variations of the sound pressure in the vicinity of the sensing element due to the thermal agitation of the surrounding gas molecules. In an ideal sensor the contribution of other noise sources such as the sensor mechanical noise is due to the thermal agitation of the molecules of the sensing element itself, or otherwise the sensor pre-amplifier noise should be negligible. This is the case indeed both for sensitive compact microphones used in PAS and for QTF's used in QEPAS. This means that the detection threshold of both sensors cannot be further improved, and the only way to lower the NEA is to increase the excitation power. With semiconductor lasers operating in the C- or L-band telecommunication range the power can be increased to watts level by using an Erbium-doped fiber amplifier, [see M. Webber, et. al., Appl. Opt., 42 (12), 2119-2126 (2003)] but this solution has an unacceptably high price. Another way to increase the excitation beam power by placing the photo-acoustic cell inside the resonant cavity of a laser is obviously not applicable to DFB semiconductor lasers, and it would not be practical even with an external cavity diode laser because the power buildup is rather small in the lossy cavities of such lasers. The last remaining possibility would seem to be to increase the excitation power by intensity enhancement of the DFB laser beam in an optical power buildup cavity (OPBC). This method has been used since about 1980 in numerous laboratory atomic spectroscopy experiments. Despite these demonstrations, the first (and the only) experiment to our knowledge that used OPBC to deliver enhanced optical power to a photo-acoustic cell has been reported by Rossi and co-workers [A. Rossi, et. al., (2005), Appl. Phys. Lett., 87, 041110 (2005)]. They reported a 100 times PAS signal increase matching a 100-fold optical power buildup in the cavity with 99.0% mirrors reflectivity. However, this work cannot be considered as showing encouraging prospects for building industrial gas OPBC-PAS sensors for several reasons. First of all, the NNEA of 1.3×10−9 cm−1 W/√Hz that is shown by the data in [A. Rossi, et. al., (2005), Appl. Phys. Lett., 87, 041110 (2005)] was not significantly better than the value of 2×10−9 cm−1 W/√Hz demonstrated with “traditional” PAS, [see V. Kapitanov, et. al., Appl. Phys. B, 90, 235-241 (2008)] where a semiconductor laser was used with no OPBC enhancement. A second important reason is that the method of locking the diode laser radiation to the cavity described in the Rossi paper was only marginally effective even in a perturbation-free laboratory environment. The unstable and unreliable operation of the diode to cavity lock also resulted in very poor stability of the buildup intensity dependence as a function of time as one can see from FIG. 2 in the Rossi reference.
The most recent development in OPBC-PAS with semiconductor lasers is reported in U.S. Pat. No. 7,263,871 by Selker and co-workers. This patent teaches how a substantial power buildup of a semiconductor diode laser can be achieved using cavities of various configurations in combination with a resonant acoustic cell inside the cavity. Several methods of keeping the semiconductor laser in resonance with the optical cavity are also described in the patent, which include both electronic methods and those that take advantage of optical feedback.
The prior art discussed above describes systems for the photo-acoustic measurement of optical absorption which use passive optical cavities with a photo-acoustic sensor inside the cavity to enhance the power of the laser source by the effect of optical power buildup and to thereby increase the photo-acoustic effect.
OPBC-PAS systems currently known split into two distinct families:                i) Systems that use a chopped laser beam, which is coupled to the OPBC, such as the one described in the reference A. Rossi, et. al., (2005), Appl. Phys. Lett., 87, 041110 (2005). The optical power circulating within the cavity is modulated in amplitude. In order to maintain the laser in resonance with the cavity peak, a small frequency dither with an amplitude equal to a small fraction of the OPBC resonance peak width is applied to the laser, and the thus obtained derivative signal is used for locking. An unavoidable drawback of such approach is that the locking cannot be done during periods when the laser is off. So the cavity drives itself out of resonance during every “laser off” period. After the laser is turned on again, a large error signal results in over-reaction of the locking system, and the lock can be lost again because of this over-reaction. As a result such systems have low immunity to external perturbations, high instability of the buildup power, and are essentially useless for applications in field useable systems.        ii) Systems that constantly maintain a lock of the laser to the cavity, such as described in U.S. Pat. No. 7,263,871. The locking in such systems in very robust, especially when high-speed modulation-free locking methods are used. In such systems the circulating power is kept at a nearly constant level, and thus they can only use wavelength modulation of the laser. With the cavity being locked to the laser, the cavity mirrors oscillate, and the sound wave resulting from this oscillation is the source of a strong background signal.        
The present invention features the best of both approaches by combining a very high reliability and robustness of the cavity lock to the laser, with a high efficiency photo-acoustic excitation which is not accompanied by the intense background sound.